In the 8 week old embryo, the urethral plate thickens on its ventral side and disintegrates in its interior. This forms the secondary urethral groove (sulcus urogenitalis definitivus) which deepens the primary groove. This definitive urethral groove is lined partly with ectoderm and partly with endoderm. It is continuous with the phallic part of the urogenital sinus. From week 9, it is possible to tell the sex of the fetus with certainty.